1. Field of the Invention
This invention relates to a magnetic field source for use in a currency detector, and more particularly to an alternating magnetic field source.
2. Description Relative to the Prior Art
The sensing of the magnetic ink used in currency for the detection of counterfeit bills is known in the art. Examples may be found in U.S. Pat. No. 4,458,143 issued in the name of Gitlis, U.S. Pat. No. 4,114,804 issued in the names of Jones and Sherman, and U.S. Pat. No. 4,764,725 in the name of Bryce. Similarly, apparatus for reading magnetic ink characters on checks has also previously been disclosed, as may be found in U.S. Pat. No. 4,668,913 issued in the name of Vinal.
The above prior art is characterized by a single bill or check passing in close proximity or in actual contact with a magnetic ink detection apparatus. There exists the need, however, for a magnetic ink detector that is neither in contact with a detector nor immediately adjacent to it but that is responsive to the magnetic field associated with a stack of currency. This need to respond to the magnetic field of currency arises for purposes such as detection of stacked currency hidden in luggage or in sealed opaque packages. The requirement mandates a detector that is responsive to magnetic material yet may be as far as 4 to 5 inches away from the package containing currency.
In detecting the presence of a stack of unobservable currency, it is necessary that the detector distinguish the money from other magnetic objects that may be in the package. A unique signature for a stack of currency may be deduced from the coercivity of the magnetic material comprising the magnetic ink. The detection of this unique signature necessitates measuring the hysteresis loop of the magnetic media of the currency. This measurement then allows the determination of the coercivity of the magnetic media.
As is known in the art, measurement of the hysteresis loop of a magnetic material requires the application of a uniaxial alternating magnetic field to the material. Such fields may be generated by means of an AC current through a coil, as taught for example in U.S. Pat. No. 3,359,495. In practice, the magnitude of the field necessary for measuring the coercivity of a stack of currency not in contact with the coil would generally require a large, heavy coil consuming a large amount of ac power. Such a heavy, high power coil would be inconvenient and burdensome to use as a hand held unit. However, the need has been felt to make available a magnetic field generating apparatus as well as a currency field detector which can be implemented as a hand held unit.